1. Field of the Invention
The present invention generally relates to semiconductor devices and methods of fabricating these devices, and more particularly, to a semiconductor device for use in high-frequency amplification and a method of fabricating the same.
2. Description of the Related Art
A field effect transistor is known as a semiconductor device suitable for amplification in a high-frequency band such as a microwave band, semi-millimeter wave band or millimeter band. A parasitic capacitance between the source and gate of the transistor and another parasitic capacitance between the drain and gate thereof serve as factors that prevents high-frequency operations. These parasitic capacitances are classified into intrinsic capacitance and extrinsic capacitance. FIG. 1 shows intrinsic and extrinsic capacitance. Cgs(int) and Cgd(int) denote gate-source intrinsic capacitance and gate-drain intrinsic capacitance, respectively. These capacitances are formed inside a semiconductor substrate 10. Cgs(ext) and Cgd(ext) denote gate-source extrinsic capacitance and gate-drain extrinsic capacitance, respectively. These capacitances are formed outside of the semiconductor substrate 10. It is desired to reduce the parasitic capacitances as small as possible in order to achieve high-frequency operation.
The extrinsic capacitances Cgs(ext) and Cgd(ext) are relatively large. Therefore, it is particularly desired to reduce the extrinsic capacitances. The extrinsic capacitances may be reduced by designing layout so that no member having a high dielectric constant exists in the vicinity of the gate electrode. A proposal based on the viewpoint mentioned above is described in, for example, Japanese Unexamined Patent Publication No. 3-35536. According to the proposal, a gap is defined around the gate electrode, and a shield electrode is provided around the gate electrode via the gap.
However, a complex production process is needed to define the gap around the gate electrode. Further, the gap may weaken the mechanical strength of the layer structure. For example, external shock applied to the device may easily destroy the gap and short-circuit the shield electrode and the gate electrode.